A Eu³⁺-Eu²⁺ ion redox shuttle imparts operational durability to Pb-I perovskite solar cells

The components with soft nature in the metal halide perovskite absorber usually generate lead (Pb)⁰ and iodine (I)⁰ defects during device fabrication and operation. These defects serve as not only recombination centers to deteriorate device efficiency but also degradation initiators to hamper device lifetimes. We show that the europium ion pair Eu³⁺-Eu²⁺ acts as the "redox shuttle" that selectively oxidized Pb⁰ and reduced I⁰ defects simultaneously in a cyclical transition. The resultant device achieves a power conversion efficiency (PCE) of 21.52% (certified 20.52%) with substantially improved long-term durability. The devices retained 92% and 89% of the peak PCE under 1-sun continuous illumination or heating at 85°C for 1500 hours and 91% of the original stable PCE after maximum power point tracking for 500 hours, respectively.